1. Field of Invention
The present invention is directed to the arts of recycling old corrugated container (OCC), more particularly to a process for the making of recycled linerboard or Kraft paper from OCC having strength properties comparable to those made using virgin unbleached Kraft pulp (UKP).
Such recycled linerboard or Kraft paper finds particular application in the inexpensive substitutes for linerboard or Kraft paper made using expensive virgin UKP.
The conventional arts of recycling OCC employ the method of fiber fractionation to separate long fibers (UKP) from OCC furnish. But due to inherent inefficiency of screen-fractionators being used, recovery of UKP content to the level required for high quality linerboard or Kraft paper has not been possible and, therefore, a large amount of virgin UKP had to be added in order to obtain strength properties demanded in the market.
This forced to waste valuable UKP in OCC and consume virgin UKP made from natural timber resources. Thus it would be beneficial to replace virgin UKP by recovered UKP from OCC and eliminate completely the need for using virgin UKP for the making of recycled linerboard or Kraft paper.
2. Description of Prior Arts
A typical stock preparation process for recycled linerboard or Kraft paper using OCC is roughly described by the FIG. 1. A high consistency furnish (4xcx9c8%) is formed at the pulper where OCC is defibered by water. Heavy contaminants in OCC furnish are eliminated by high-density cleaners and large size but relatively light contaminants are eliminated by coarse screens. In addition to cleaning and screening, high-density cleaners and coarse screens perform defibering of remaining OCC fractions.
The corrugating medium glued between the two facings of linerboards is composed mainly of semichemical hard wood pulps having fiber length shorter than 2 millimeters. Although unbleached Kraft pulps (UKP) having fiber length longer than 3 millimeters occupies over 80% of linerboard, the UKP content in OCC furnish is reduced to approximately 55% because of the low quality fibers in corrugated medium that takes about one third of the OCC weight.
More water is added to the OCC furnish to make the consistency of the furnish below 1% and sent to fine screens to eliminate small size contaminants from OCC furnish as rejects.
Then accepts from fine screens are sent to fractionators to divide OCC furnish into two separate streams, long fibers and short fibers.
Long fiber stream goes through lightweight cleaners and forward cleaners where lightweight contaminants are eliminated. And at the disk thickener water is removed to make the consistency high enough to be refined by dispersion refiner.
Then the remaining contaminants such as wax are further dispersed into small particles by heating tubes and sent to paper machine wire.
The increase in UKP content in the long fiber stream achieved by fractionators is not sufficient to meet with strength requirement of recycled linerboard or Kraft paper. This forces the addition of virgin UKP to the long fiber stream.
The short fiber stream from fractionators are further cleaned by lightweight cleaner and water is removed by disk thickener before being sent to paper machine that makes corrugating medium. Normally, low quality recycled fiber is further added to the short fiber stream before it is sent to a paper machine that makes corrugating medium.
In the arts of OCC recycling described above, deterioration in strength properties of linerboard or Kraft paper made from fractionated OCC furnish has been taken as unavoidable. Therefore, manufacturing of recycled linerboard or Kraft paper required addition of a large amount of virgin UKP to achieve the level of strength properties demanded in the market.
In one embodiment, the invention is directed to a stock preparation process for the making of recycled linerboard or Kraft paper, wherein a furnish is prepared by pulping OCC, high-density cleaning, coarse screening, fine screening, fractionating, lightweight cleaning, forward cleaning, disk thickening, dispersion refining, dispersing wax into small particles, and adding virgin UKP, characterized in:
Separating linerboard from corrugated medium before pulping.
In another embodiment, the invention is directed to a stock preparation process for the making of recycled linerboard or Kraft paper, separating linerboard is achieved by shredding and cutting OCC into pieces and then by float-separating linerboard pieces in water.
In still further embodiment, the invention is directed to a stock preparation process for the making of recycled linerboard or Kraft paper, wherein separating linerboard is achieved by crushing OCC into pieces by agitating OCC in water.
In still further embodiment, the invention is directed to a stock preparation process for the making of recycled linerboard or Kraft paper, wherein separating linerboard is achieved by an auxiliary pump and/or gate valve installed between a pulper and a short fiber chest.
In still further embodiment, the invention is directed to a stock preparation process for the making of recycled linerboard or Kraft paper, wherein elimination of micro-fibrils from recycled furnish is achieved by disk thickener and/or fractionators.
In still another embodiment, the invention is directed to a stock preparation process for recycled linerboard or Kraft paper, wherein recovery of long fibers from reject streams of fine screens is achieved by non-refining disperser.
A further embodiment discloses a process of stock preparation for recycled linerboard or Kraft paper characterized in the following steps:
(a) Cutting and/or Crushing OCC into pieces,
(b) Separating linerboard pieces from corrugated medium pieces before pulping linerboard pieces,
(c) Removing corrugated medium pieces remaining in linerboard pieces by pulping corrugated medium pieces first and then by sending defibered corrugated medium to short fiber chest by actuating auxiliary pump and/or gate valve.
(d) Adding effective amounts of NaOH to the variable speed pulper immediately after removing corrugated medium.
(e) Eliminating micro-fibrils from linerboard or Kraft paper furnish by disk thickener and/or fractionators,
(f) Minimizing long fiber loss by recovering long fibers from reject streams of fine screens by non-refining disperser,
(g) Applying dry strength additives to linerboard or Kraft paper furnish.
(h) Maintaining a pH of furnish at the head box of paper machine at least 5.5 and at highest 6.5.